The interaction of X-rays and immunity in the killing of tumor cells is being studied using the C3H mouse ascites sarcoma BP8. The tumor grows in C3H/DMC hybrids from inocula of less than 10 cells. Host resistance against the tumor can be raised in these mice by inoculation with lethally irradiated tumor cells. The immunity has been quantitated by measuring survival after viable cell challenge or by using a method in which tumor cells are prelabeled with the thymidine analog 125-I-labeled iododeoxyuridene and followed after injection into mice. Loss of radioactivity is monitored by whole-body gamma counting and this loss can be correlated with loss of tumor cells which results from either immune reactions or other therapeutic measures. These methods will be used to quantitate tumor cell death caused by weak host resistance, sublethal X-rays or by combinations of both means of cell killing.